1. Field of the Invention
This invention relates to a control and supervisory signal transmission system, and more particularly to a control and supervisory signal transmission system having a simple structure in which an intermediate station is provided in place of a controller and a parent station, wherein a parallel control signal from the intermediate station is converted into a serial signal and the serial signal is transmitted to a controlled unit in a remote device, wherein the serial signal is converted into a parallel signal to drive the device and a parallel supervisory signal of a sensor unit which detects the status of the device are converted into a serial signal and the serial signal is transmitted to the intermediate station, and wherein the serial signal is converted into a parallel signal and provided to the intermediate station, where the supervisory and control signals are superimposed on a clock signal having a power supply signal superimposed on it.
2. Description of the Related Art
In automatic control fields, technologies are widely used in which a controller such as a sequence controller, programmable controller, or computer sends a control signal to drive and control a large number of controlled devices (motors, solenoids, solenoid valves, relays, thyristors, and lamps, for example) at remote locations and supervisory signals are transmitted and provided from a sensor unit to detect a state of each device (the on/off state of read switches, micro-switches, push-button switches, for example) to the controller.
In these technologies, a number of lines such as a power supply line, control signal line, and a ground line, are conventionally used to connect the controller with the controlled unit and with sensor units. With the downscaling of controlled devices in recent years, wiring in a high-density layout of components of the devices has become difficult, and space for wiring has reduced and become costly.
In order to solve these problems, various technologies have been proposed (in Japanese Patent Laid-Open No. 01-072623, No. 01-089839, and No. 03-006997, for example). According to the invention described in Japanese Patent Laid-Open No. 03-006997, an input unit and output unit are connected to a parent station and the parent station provides a clock signal superimposed on a power supply onto a common data signal line. Thus fast bidirectional signal transmission between the controller and the controlled units and between the controller and the sensor units can be achieved with a simple structure. That is, the system can be configured with a reduced number of lines, the costs of the wiring can be reduced, the connections in child stations can be simplified, and addresses can flexibly be allocated to the child stations, and therefore child stations can be flexibly added or deleted at any locations requiring addition or deletion of such child stations.
A control and supervisory signal transmission system has been proposed as an advanced version of this technology (Japanese Patent Laid-Open No. 2003-199178). According to that invention, a signal from the controller to controlled units (hereinafter called a control signal) and a signal from a sensor unit to the controller (hereinafter called a supervisory signal) can be superimposed on a clock signal. In particular, the control and supervisory signals can be provided onto a data signal line and transmitted bidirectionally at the same time. As a result, the need for providing separate periods for transmitting a control signal and supervisory signal on the data signal line can be eliminated, the signal transmission rate can be made twice as faster, and a control signal can be transmitted to small wiring spaces in controlled units to obtain a supervisory signal.
According to the control and supervisory signal transmission system described in Japanese Patent Laid-Open No. 2003-199178, control and supervisory signals can be transmitted between a parent station and child stations on various types of sites to adapt to an extremely wide range of requirements. However, at sites where it is desired that two power supply lines be eliminated by superimposing power supply on two data signal lines, a small system is often demanded.
We has investigated such demands and found that, in most sites where a small system is demanded, a system that transmits an input signal (supervisory signal) to a remote output device (controlled unit) in simple one-to-one correspondence, that is, a system that has a simple structure, is easy to maintain, and inexpensive, is demanded.
We has further studied and found that the controller and a parent station can be replaced with a simple structure when an input and output are transmitted in the same clock cycle (or period) in simple one-to-one correspondence as mentioned above and thus a small, easy-to-maintain, and inexpensive system can be achieved. Also, the inventor has found that the same clock cycle can be divided into two periods, an input period followed by an output period, and the input and output are superimposed on signals in the input period and output period to be transmitted, respectively, to achieve a simple structure that can replace the controller and a parent station can be adopted and thus a small, easy-to-maintain, and inexpensive system.